A team from the International Atomic Energy Agency is in Japan visiting the Fukushima Daiichi nuclear power plant. The visit comes a week after reports emerged that large amounts of radioactive water had leaked from reservoirs where it was being stored.

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And I'm Steve Inskeep. A team from the International Atomic Energy Agency is visiting a Japanese nuclear power plant this week. The team has been checking on that plant for some time - the plant that melted down. Three of the reactors there melted down after the 2011 earthquake and tsunami. And they arrive amid reports that a large amount of radioactive water had leaked from the plant. NPR science correspondent Geoff Brumfiel is looking into the implications and has today's Business Bottom Line. Welcome to the program, Geoff.

GEOFF BRUMFIEL, BYLINE: Thanks. Nice to be here.

INSKEEP: It's good to say good morning for the first time here. Why is radioactive water leaking?

BRUMFIEL: Well, as you may remember, these plants melted down in 2011 and what's happened is there's still fuel inside the reactor cores and that fuel still needs to be kept cool.

INSKEEP: Mm-hmm.

BRUMFIEL: So they've had to bring in water from outside and pour it in and continue putting in water. That water flows down into the basement of the plant. They pump it out, decontaminate it and store it. And it's in that complex process that these leaks have been found.

INSKEEP: Just so I understand, when you say flows down into the basement of the plant, it's not flowing into a tank? They're actually flooding the basement in order to keep the uranium from going any hotter than it is?

BRUMFIEL: Well, the problem here is this plant was heavily damaged and no one's ever been able to repair it. So it's still broken, basically. There's lots of broken pipes and cracks and things. And nobody really knows what the situation is. But when you put water in, it leaks out.

INSKEEP: So they're sending water in. They don't really know where it's going and some of it's coming out. How serious is it?

BRUMFIEL: Well, the situation with the leaks is - it's a little unclear. So just to sort of expand on what I said before, when they bring the water out they have to decontaminate it and then store it somewhere. And they've been using metal tanks on site. But the problem is that these tanks have been filling up because groundwater has also been coming into the basements.

INSKEEP: OK.

BRUMFIEL: So instead of using tanks they recently switched to reservoirs. And it's these reservoirs that have been found to be leaky. They are just earthen pits that have been lined with sheets of plastic and the plastic may have torn, or it may just leak. Nobody really knows.

INSKEEP: Just - I mean, we must be talking about many, many, many, many thousands of gallons of water at this point that have been contaminated in some way.

BRUMFIEL: Yes, we certainly are. And in fact, I mean, there are some estimates that these leaks may have leaded as much as tens of thousands of gallons of water.

INSKEEP: And is the situation that the officials find themselves in that they know that there is now radioactive water in the ground but they're not able to identify which of the many places that it could be coming from it actually is coming from?

BRUMFIEL: No. They know where it's coming from, all right.

INSKEEP: OK.

BRUMFIEL: Because there are monitors around all these reservoirs. So they know which ones are leaking. The thing is they don't know how much has leaked. The problem here is that they have some gauges to measure the height in the reservoirs of the water level. It looks like it's dropped. If you take it straight at face value that means tens of thousands of gallons have leaded.

But it may not be that much. There's a margin for error and they're a little uncertain right now.

INSKEEP: A couple of quick questions. Is this leakage a sign of incompetence?

BRUMFIEL: Well, that's a very good question. I mean, I think even TEPCO would admit that they probably shouldn't have been using plastic sheeting to hold, you know, this contaminated water.

INSKEEP: That's the utility here? OK. All right.

BRUMFIEL: Yes, sorry. The Tokyo Electric Power Company.

INSKEEP: That maybe plastic sheeting was not sufficient. OK. And is this dangerous?

BRUMFIEL: Well, this water's been partially decontaminated already. So it's not as dangerous as if it were coming straight from the core. But, I mean, whenever you have an uncontained loss of radioactive material in any situation, I think it's something you'd prefer to avoid.

INSKEEP: When utility officials look at this, when government officials look at this, when the International Atomic Energy Agency looks at this, do they see an end? A moment when this situation will be cleaned up?

BRUMFIEL: Well, the moment when this situation will be cleaned up is decades away. I mean, you have to remember these plants still have the fuel inside. There's still spent fuel that was being stored above the reactors and that's still in there. And no one can really get to it right now. The area is just too radioactive. So we're going to have to wait for all the fuel to cool down and then we're going to have to figure out how to go in there and get it out.

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